Hugo Ortner

Synorogenic clastics were deposited on the growing orogenic wedge of the Northern Calcareous Alps from the Early Cretaceous to the Miocene. Sedimentation of the Gosau Group took place between the Turonian and the Late Eocene. Generally, the Gosau Group is subdivided into the Lower Gosau Subgroup, dominated by terrestrial to near-shore deposits, and the Upper Gosau Subgroup consisting of deep marine clastics. The Gosau deposits monitor the tectonic evolution in time during deposition.

The Gosau Group of the Muttekopf area is preserved in a large WSW-trending syncline (Muttekopf-Sinnesbrunn-syncline; Niederbacher 1982) in the Hauptdolomit of the Inntal Nappe, one of the large tectonic units of the western Northern Calcareous Alps. Three-dimensional analysis of the succession was facilitated by excellent outcrop conditions.

In the Muttekopf area the Lower Gosau Subgroup is only preserved in the easternmost part of the outcrops (Plattein area). In the western part, the thickness of these deposits is reduced and they were locally removed erosively before onset of sedimentation of the Upper Gosau Subgroup. A NW-trending dextral fault separating the Plattein area from the more western outcrops is interpreted to have been active between the Coniacian und Santonian, leading also to vertical movement, that prevented erosion in the Plattein area. More to the west, another dextral NW trending fault east of Schlenkerspitze (Fig. 1) with a lateral offset of about 1km is sealed by sediments of the Upper Gosau subgroup. In areas between dextral faults, folds with a wavelength of about 100 m with NE-trending fold axes are truncated and sealed by sediments of the Upper Gosau subgroup.

The Upper Gosau Subgroup in the Muttekopf area consists of a succession of marls, sandstones and breccias that can be subdivided in three fining-upward megacycles (Ortner 1994). The first and the second megacycle were deposited from Late Santonian to Early Maastrichtian times. The first megacycle is restricted to the northern limb of the syncline and thins towards the south and west. Most of the sediments in the Muttekopf area are part of the second megacycle.

The profile across the syncline in the second megacycle shows several progressive unconformities (cf. Riba 1976) in the southern, steep limb of the fold, that become conformities in the northern flat limb of the syncline. Growth synclines are characterized by zones of growth offlap-onlap (Ford et al. 1997), as seen in the profile (Fig. 2, Detail). The wedging of the first megacycle towards the south could therefore be interpreted as growth onlap. Both the axis of the syncline in the Gosau deposits and in the anticline in the south in the underlying Hauptdolomite trends WNW.

Two conclusions can be drawn from these observations: 1) Structures of an older deformational event are sealed by Gosau sediments. During this event, dextral faulting along NW-trending faults was accompanied by minor folding with NE-trending axes.

2) The Upper Gosau Subgroup of the Muttekopf area was deposited during active shortening, the area of deposition has to be interpreted as piggy back basin. The asymmetry of the cross sections is not due to hidden normal faults (as suggested by Ortner 1994), but is the effect of deposition onto growing folds and is in accordance with widespread large scale slumping, deposition of megabreccias and with cannibalisation of large amounts of Gosau sediments.